In recent years, the market share of sales of photographic image sensors that are mounted in cameras such as television cameras and video cameras which use charge coupled device (hereinafter CCD) arrays as the image sensor has rapidly increased, and a high resolution over the entire image field has been strongly desired. Hence, photographic lenses for such cameras are required to have a uniformly high optical performance over the entire image field for all photographing conditions.
Meanwhile, numerous extender optical systems have been proposed for insertion between a photographic main lens and a camera body for changing the combined focal length (i.e., with the extender optical system in place) to the long focus side. Such extender optical systems also are required to have a high optical performance so that images are not degraded by use of the extender optical system.
Examples of such extender optical systems are disclosed in Japanese Laid-Open Patent Applications H10-133248, H11-258489 and others. However, in the extender optical systems described above, the reduction of chromatic aberration and lateral color is insufficient at image magnifications of 1.1 or higher. Thus, there is a need for an extender optical system that provides higher quality images, especially images having reduced chromatic aberration and lateral color at magnifications of 1.1 or higher.
Extender optical systems having an anti-vibration mechanism for preventing the blurring of images caused by vibrations (as often occur when a camera is hand-held) are known in the art. For example, the aforementioned laid-open patent applications disclose anti-vibration mechanisms that move the extender optical system as a whole or part of a plurality of lenses as a unit in the direction perpendicular to the optical axis. However, because many lenses are made to move back and forth as a unit, the weight of the objects being driven becomes large, resulting in a bulky driving system.
Moreover, in the prior art of the second laid-open patent application mentioned above, the extender optical system has a structure suitable to a photographic main lens having a relatively long back focus, making it difficult for use in a system with a short back focus, such as a back focus of 40 mm or less.
The present invention is an extender optical system that is suitable for use in television cameras and video cameras, and is to be mounted between a photographic main lens and a camera body in order to change the focal length to the long focus side. In particular, the present invention is an extender optical system that is especially suited for use with television cameras and video cameras that require a relatively long back focus so as to accommodate a color separation optical system within the front stage of the camera main body.
A first feature of the present invention is that it provides an extender optical system that maintains a resolution of the main camera lens by more favorably correcting chromatic aberration and lateral color than in prior art extender optical systems. A second feature of the present invention is that an anti-vibration mechanism can be used without making the drive system bulky. A third feature of the invention is that the extender optical system can be used with a photographic main lens that, by itself, has a rather short back focus.